1. Field of the Invention
This invention relates in general to subsea wellhead equipment, and in particular to a casing or tubing hanger having a metal-to-metal seal.
2. Description of the Prior Art
A typical subsea wellhead assembly includes a wellhead housing mounted within a permanent guide base that is supported on the ocean floor by a temporary guide base. Large diameter conductor pipe is secured to the wellhead housing and extends downward into the earth a short distance. A wellhead is mounted inside the wellhead housing and to a permanent guide base which mounts on the top of the temporary guide base. Surface casing secured to the wellhead extends a few hundred feet down into the well. The top of the wellhead is connected to pressure equipment and risers that extend to a drilling vessel at the surface. As the well is drilled deeper, a first string of casing may be set to a certain depth. Subsequently, a second string of casing may be set.
In a typical installation, the casing hanger includes a casing hanger body which is secured to the upper end of the casing string. The body is supported on an annular shoulder in the wellhead. A seal and a locking means are located in annular clearances between the casing hanger body and wellhead bore. The seal normally includes an elastomeric ring which is compressed by compression rings between the casing hanger body and the wellhead bore. The locking means includes a split ring and/or various wedges, which are normally actuated by rotation of a running tool to lock the elastomeric seal in compression and to lock the casing hanger in the wellhead. Wickers, which are small parallel grooves, may be located in the wellhead bore for engagement by the split ring or wedges. The locking means provides support for the casing hanger.
It is important to be assured that the locking means has properly landed on the landing shoulder and actuated. In one prior art technique, spring clips are used to retain the locking means in place prior to actuation. This requires boring holes through the wellhead, which is a disadvantage. Also, if the casing hanger is later pulled and another ran, the spring clips could not be used again.